1. Field of the Invention
The invention is directed to a novel sealing composition and a method for improving performance of an electrophoretic display, especially at high temperatures.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a dielectric solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other and separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology is disclosed in U.S. Pat. No. 6,930,818 (corresponding to WO 01/67170), U.S. Pat. No. 6,672,921 (corresponding to WO 02/01281), U.S. Pat. No. 6,933,098, (corresponding to WO 02/65215) and U.S. Publication No. 2002-0188053; all of which are incorporated herein by reference.
A typical microcup-based display cell is shown in FIG. 1. The cell (10) is sandwiched between a first electrode layer (11) and a second electrode layer (12). A primer layer (13) is optionally present between the cell (10) and the second electrode layer (12). The cell (10) is filled with an electrophoretic fluid and sealed with a sealing layer (14). The first electrode layer (11) is laminated onto the sealed cell, optionally with an adhesive (15).
The display panel may be prepared by microembossing or photolithography as disclosed in WO 01/67170. In the microembossing process, an embossable composition is coated onto the conductor side of the second electrode layer (12) and embossed under pressure to produce the microcup array.
The embossable composition may comprise a thermoplastic, thermoset or a precursor thereof which may be a multifunctional acrylate or methacrylate, vinylbezene, vinylether, epoxide and the like, or an oligomer or polymer thereof. In one alternative, multifunctional acrylates and their oligomers are used. A combination of a multifunctional epoxide and a multifunctional acrylate is also very useful to achieve desirable physico-mechanical properties. A crosslinkable oligomer imparting flexibility, such as urethane acrylate or polyester acrylate, is usually also added to improve the flexure resistance of the microcups prepared from the microembossing process. The composition may contain an oligomer, a monomer, additives and optionally a polymer. The glass transition temperature (g) of the embossable composition may range from about −70° C. to about 150° C., preferably from about −20° C. to about 50° C.
The microembossing process is carried out at a temperature higher than the Tg of the embossable composition. A heated male mold or a heated housing substrate against which the mold presses may be used to control the microembossing temperature and pressure.
The mold may be released during or after the embossable composition is hardened to reveal an array of microcups (10). The hardening of the embossable composition may be accomplished by cooling, solvent evaporation, cross-linking by radiation, heat or moisture. If the curing of the embossable composition is accomplished by UV radiation, UV may radiate onto the embossable composition through the transparent conductor layer. Alternatively, UV lamps may be placed inside the mold. In this case, the mold must be transparent to allow the UV light to radiate through the pre-patterned male mold on to the embossable composition.
A thin primer layer (13) is optionally precoated onto the conductor layer to improve the release properties of the mold. The composition of the primer layer may be the same or different from the embossing composition.
In general, the dimension of individual cells may be in the range of about 102 to about 106 μm2, preferably from about 103 to about 105 μm2. The depth of the cells may be in the range of about 3 to about 100 microns, preferably from about 10 to about 50 microns. The ratio between the area of openings to the total area of the microcup array may be in the range of from about 0.05 to about 0.95, preferably from about 0.4 to about 0.9. The width or length of the openings may be in the range of from about 15 to about 450 μm, preferably from about 25 to about 250 μm, from edge to edge of the openings.
The microcups are filled with an electrophoretic fluid and sealed as disclosed in WO 01/67170, U.S. Publication No. 2002-0188053 or U.S. Publication No. 2004-0112525. The sealing of the microcups may be accomplished in a number of ways. For example, it may be accomplished by a two-pass sealing process involving overcoating the filled microcups with a sealing composition comprising a solvent and a sealing material selected from the group consisting of thermoplastic elastomers, polyvalent acrylates or methacrylates, cyanoacrylates, polyvalent vinyls including vinylbenzene, vinylsilane and vinylether, polyvalent epoxide, polyvalent isocyanate, polyvalent allyl, oligomers or polymers containing crosslinkable functional groups and the like. Additives such as a polymeric binder or thickener, photoinitiator, catalyst, filler, colorant or surfactant may be added to the sealing composition to improve the physico-mechanical properties and the optical properties of the display. The sealing composition is incompatible with the electrophoretic fluid and has a specific gravity lower than that of the electrophoretic fluid. Upon solvent evaporation, the sealing composition forms a conforming seamless seal on top of the electrophoretic fluid. The sealing layer may be further hardened by heat, radiation or other curing methods. In one embodiment, sealing is accomplished with a composition comprising a thermoplastic elastomer. Examples of thermoplastic elastomers include polyurethanes, polyesters, tri-block or di-block copolymers of styrene or a-methylstyrene and isoprene, butadiene or ethylene/butylene, such as the Kraton™ D and G series from Kraton Polymer Company. Crystalline rubbers such as poly(ethylene-co-propylene-co-5-methylene-2-norbornene) and other EPDMs (Ethylene Propylene Diene Rubber terpolymers) from Exxon Mobil have also been found useful.
Alternatively, the sealing composition may be dispersed into an electrophoretic fluid and filled into the microcups (i.e., one pass sealing process). The sealing composition is incompatible with the electrophoretic fluid and is lighter than the electrophoretic fluid. Upon phase separation and solvent evaporation, the sealing composition floats to the top of the filled microcups and forms a seamless sealing layer thereon. The sealing layer may be further hardened by heat, radiation or other curing methods.
The sealed microcups finally are laminated with the first electrode layer (11) which may be pre-coated with an adhesive (15) such as a pressure sensitive adhesive, hot melt adhesive, moisture or UV curable adhesive.
The sealing layer formed seamlessly encloses and isolates the electrophoretic fluid within the microcups. It also provides good adhesion between the microcup-based cells (10) and the first electrode layer (11) and enables an efficient roll-to-roll production of the displays.
In order to improve the switching performance, it is disclosed in U.S. Publication No. 2004-0085619, that a conductive material, in the form of particles, may be added to the sealing composition. Suitable conductive materials include organic conducting compounds or polymers, carbon black, carbonaceous particles, graphite, metals, metal alloys and conductive metal oxides.
It is also disclosed in the same co-pending application that a high absorbance dye or pigment may be added to the adhesive layer to improve the display performance. Suitable dyes or pigments may have an absorption band in the range of 320-800 nm, preferably in the range of 400-700 nm. Useful dyes and pigments include metal phthalocyanines or naphthalocyanines (wherein the metal may be Cu, Al, Ti, Fe, Zn. Co, Cd, Mg, Sn, Ni, In, V or Pb), metal porphines (wherein the metal may be Co, Ni or V), azo (such as diazo or polyazo) dyes, squaraine dyes, perylene dyes and croconine dyes.
While the microcup-based electrophoretic displays have shown good display performance and involve low cost for their manufacture, there are still features that can be further improved.
The whole content of each document referred to in this application is incorporated by reference into this application in its entirety.